The present invention relates to a rotary grinding mill which incorporates means for injecting a fluid into the mass of material being ground.
The invention also relates to a feed arrangement for a rotary grinding mill.
Conventional rotary mills include a cylindrical drum rotated about a generally horizontal axis. The rotating drum is fed with particulate material such as a slurry or powder, the rotation of the drum being at one half to three quarters of the xe2x80x9ccritical speedxe2x80x9d (ie. the minimum speed at which material at the inner surface of the drum travels right around in contact with the mill). This causes a tumbling action as the feed and any grinding media travels part way up the inner wall of the drum then falls away to impact or grind against other particles in the feed. Size reduction of the particles is thus achieved principally by abrasion and impact.
The present Applicant""s International Patent Application No. WO 99/11377 discloses a new grinding mill construction in which a rotary container is spun significantly above critical speed to form a compressed layer of the particulate material retained against the container inner surface, and shear inducing means contacting the layer to induce shearing in the layer. This has been found by the Applicant to create stirred, high shear zones in the compressed material adjacent the shear inducing means, providing a very effective grinding mechanism.
It has now been discovered by the Applicant that this arrangement provides an environment in which chemical reactions or other property-modifying operations may be conducted upon the particulate material being ground in the mill.
The mill described in WO 99/11377 is gravity-fed, the slurry or powder of particulate material entering one end of the rotating grinding container via a feed tube. After grinding, smaller particles migrate radially inwards and pass inside an annular discharge opening it the other end of the mill.
This arrangement works well where gravity feed of the mill is suitable, but would require a sophisticated seal arrangement to allow adaptation of the mill to pressure feed, in view of the high rotary speed of the mill and the abrasive environment.
In one form, the present invention provides a grinding mill for particulate material, including a rotary container having an inner surface, feed means for feeding the particulate material to the container, means rotating the container at a sufficiently high speed that the particulate material forms a layer retained against the inner surface throughout its rotation, and shear inducing means contacting said layer so as to induce shearing in said layer, the grinding mill further including means for introducing a fluid into said layer to contact the particulate material.
Preferably the fluid is injected into said layer in the vicinity of said shear inducing means, and desirably is introduced through said shear inducing means.
Preferably the shear inducing means includes a shaft with at least one projection extending into the particulate layer, the shaft and projection including a passage communicating from a source of said fluid to a position in said layer.
As an alternative, the fluid can be included with the particulate material introduced to the container via said feed means.
A further form of the invention comprises a method of grinding particulate material, including feeding the particulate material to a container which has an inner surface, rotating the container at a sufficiently high speed that the particulate material forms a layer retained against the inner surface throughout its rotation, and contacting the layer with shear inducing means to induce shear in said layer, further including introducing fluid into said layer to contact the particulate material.
Preferably the fluid is injected into said layer.
Alternatively, the fluid is included with the particulate material fed to the container.
In another form, the present Invention provides a grinding mill for particulate material, including a rotary container having an inner surface, feed means for feeding the particulate material to the container, means rotating the container at a sufficiently high speed dial, the particulate material is retained against the inner surface throughout its rotation, and shear inducing means contacting said particular material so as to induce shearing wherein the shear inducing means is non rotational and is mounted on a stationary shaft which passes into the rotary container through a discharge end thereof, with mill discharge opening spacing said shaft from the rotary container, the shaft including the feed means, that introduces the particulate material into the container at a position separated from the discharge end by one or more shear zones created by the shear inducing means, such that the particulate material passes through the shear zone to reach the discharge end.
The feed means can also be used to introduce a fluid with the particulate material to the container.
It will be appreciated that the term xe2x80x9cgrinding willxe2x80x9d as used herein is not to be construed as limited to mills for the primary purpose of substantial particle size reduction, but also includes mills for the purpose of grinding, or attritioning, a freshly exposed surface an the particles before contacting with flotation chemicals or other reagents.
A further form of the invention comprises it method of grinding particulate material, including feeding the particulate material to a container which has an inner surface, rotating the container at a sufficiently high speed that the particulate material is retained against the inner surface throughout its rotation, contacting the particulate material with shear inducing means to induce shear and discharging ground material from the mill through a discharge opening at a discharge end of the container, wherein the shear inducing means is mounted on a stationary shaft passing into said container and spaced from the rotary container by the discharge opening and wherein said feed of particulate material passes through said shaft into said container at a position separated from the discharge end by one or more shear zones created by the shear inducing means.